Role play = Extra XP?

That depends on how you do it. The usual XP for good rolplaying has two problems
> GM and players never discuss their characters personality. Forcing the GM to guess at it. Those that the GM guesses correctly, typically because they are more stereotypical characters, get more roleplay XP.
> The GM needs to notice the RP. Certain PC personalities are more noticeable than others, so they get more XP.

To solve it, you need to do two things:
- Have players define their characters personality at character creation. Even a one sentence motto is enough. Letting players change this for free between sessions should be allowed to represent character development.
- At the end of the session, the GM awards the roleplay XP to whoever stuck to their defined personality. Each player either gets the full RP XP, or they get none.

I've done this in a system that awards a third of the XP this way. I only had one session where one player was acting so out of character to deny him the XP.

Why? It works.

Only if everyone is ok if some players get ahead of others in power. Some people are shy and don't speak up, it's harsh to "punish" them by only talking when they need to and not being able to stay totally in character.

Yes.

But only if characters who are mechanically optimised, and who deal the biggest numbers during combat,
get a +20% bonus to GM Fiat, favouritism, and having NPC interaction go the way they want it to go.

Good at building your character?
Get a bonus to roleplaying.

Good at roleplaying?
Get a bonus to building your character.

It's only fair.

>But what if you're good at both roleplaying AND building your character
You get both bonuses!

> +20% XP
Context required. But yes getting /small/ amounts of extra EXP for good roleplaying is pretty common.

>One guy even got salty when I wouldn't give him XP for a pvp duel.
Because those reward XP. "Defeat" doesn't mean "Kill". It usually does, but PvPing until surrender or KO counts as a defeat.

But user, good RP is always about entertaining GM (and other players) with your RP.
If you don't like idea of leaving it in arbitrary hands of the GM and subject only to his personal tastes, you might instead leave it to vote or open discussion amongst the players, provided you have mature and sensible enough group.
This also works only with giving total XP at the end of a session i nstead of rewarding individual encounters and achievements, but this is the best method of dealing with XP either way.
It does not work. Characters always develop in play in different way than any theoretical psychological background you created beforehand. Sticking to pre-made theory is not important. What's important is consistency and being able to feel the character and act it out with this feeling. RP is all about creating a good story together, not about sticking to some patterns in your head even if they DO NOT WORK in reality.

>Join a new group
>Playing FF Star wars
>The GM informs me at the end of each session they take a vocal vote on who was the best roleplayer in the group and they get between 5-10 exp
>Okay sounds neat
>Other players seem to be more focused just throwing dice at the problems the GM gives them rather than roleplaying it out
>Being a combat focused character I attempt to get more creative with how I do combat with the dice to back it up
>By the third session the GM has voted me to receive the exp every time while the others in the group have just rotated between themselves.
>I say this to the GM in private and he agrees but because he is friends with them he doesn't want to make a problem out of it
>Leave game but remain in contact with the GM because he was a cool guy

Thats been my experience

oh great whichever player jerks the DM off the most and who overacts and drama whores the most gets bonus XP

GURPS uses a mix of objective driven and roleplaying driven XP allocation. No XP for kills, but for achieving your goal. Extra XP for roleplaying.